When I make a fresh checkout of the main branch on Windows and type "pcbuild\build" it starts downloading some distributions (e.g. sqlite) and then starts building. Fine. But at some point there's a whole bunch of errors that seem to come from building zlibmodule.c. Re-running pcbuild\build then downloads some extra thing and then everything builds to completion.

First set of downloads and selected logs:

Using py -3.9 (found 3.9 with py.exe)
Fetching external libraries...
Fetching bzip2-1.0.6...       
Fetching sqlite-3.35.5.0...
Fetching xz-5.2.2...
Fetching zlib-1.2.11...
Traceback (most recent call last):
  File "C:\Users\gvanrossum\deepfreeze\PCbuild\get_external.py", line 60, in <module>
    main()
  File "C:\Users\gvanrossum\deepfreeze\PCbuild\get_external.py", line 56, in main
    extract_zip(args.externals_dir, zip_path).replace(final_name)
  File "C:\Users\gvanrossum\AppData\Local\Programs\Python\Python39\lib\pathlib.py", line 1395, in 
replace
    self._accessor.replace(self, target)
PermissionError: [WinError 5] Access is denied: 'C:\\Users\\gvanrossum\\deepfreeze\\PCbuild\\..\\externals\\cpython-source-deps-zlib-1.2.11' -> 'C:\\Users\\gvanrossum\\deepfreeze\\PCbuild\\..\\externals\\zlib-1.2.11'
Fetching external binaries...
Fetching libffi-3.3.0...
Fetching openssl-bin-1.1.1l...
Fetching tcltk-8.6.11.0...
Finished.
Using "C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\MSBuild\Current\Bin\msbuild.exe" (found in the Visual Studio installation)
Using py -3.9 (found 3.9 with py.exe)

C:\Users\gvanrossum\deepfreeze>"C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\MSBuild\Current\Bin\msbuild.exe" "C:\Users\gvanrossum\deepfreeze\PCbuild\pcbuild.proj" /t:Build /m /nologo /v:m /clp:summary /p:Configuration=Release /p:Platform=x64 /p:IncludeExternals=true /p:IncludeCTypes=true /p:IncludeSSL=true /p:IncludeTkinter=true /p:UseTestMarker= /p:GIT="C:\Program Files\Git\cmd\git.exe"
  Killing any running python.exe instances...
  Regenerate pycore_ast.h pycore_ast_state.h Python-ast.c
  C:\Users\gvanrossum\deepfreeze\Python\Python-ast.c, C:\Users\gvanrossum\deepfreeze\Include\inte
  rnal\pycore_ast.h, C:\Users\gvanrossum\deepfreeze\Include\internal\pycore_ast_state.h regenerat
  ed.
  Regenerate opcode.h opcode_targets.h
  Include\opcode.h regenerated from Lib\opcode.py
  Jump table written into Python\opcode_targets.h
  Regenerate token-list.inc token.h token.c token.py
  Generated sources are up to date
  Getting build info from "C:\Program Files\Git\cmd\git.exe"
  Building heads/deepfreeze:044e8d866f deepfreeze
  _abc.c

...

Errors:

  Compiling...
  thread.c
  traceback.c
  zlibmodule.c
C:\Users\gvanrossum\deepfreeze\Modules\zlibmodule.c(10,10): fatal error C1083: Cannot open includ
e file: 'zlib.h': No such file or directory [C:\Users\gvanrossum\deepfreeze\PCbuild\pythoncore.vc
xproj]
  adler32.c
c1 : fatal error C1083: Cannot open source file: 'C:\Users\gvanrossum\deepfreeze\externals\zlib-1 
.2.11\adler32.c': No such file or directory [C:\Users\gvanrossum\deepfreeze\PCbuild\pythoncore.vc 
xproj]
  compress.c
c1 : fatal error C1083: Cannot open source file: 'C:\Users\gvanrossum\deepfreeze\externals\zlib-1
.2.11\compress.c': No such file or directory [C:\Users\gvanrossum\deepfreeze\PCbuild\pythoncore.v
cxproj]
  crc32.c
c1 : fatal error C1083: Cannot open source file: 'C:\Users\gvanrossum\deepfreeze\externals\zlib-1
.2.11\crc32.c': No such file or directory [C:\Users\gvanrossum\deepfreeze\PCbuild\pythoncore.vcxp 
roj]
  deflate.c
c1 : fatal error C1083: Cannot open source file: 'C:\Users\gvanrossum\deepfreeze\externals\zlib-1
.2.11\deflate.c': No such file or directory [C:\Users\gvanrossum\deepfreeze\PCbuild\pythoncore.vc 
xproj]
  infback.c
c1 : fatal error C1083: Cannot open source file: 'C:\Users\gvanrossum\deepfreeze\externals\zlib-1
.2.11\infback.c': No such file or directory [C:\Users\gvanrossum\deepfreeze\PCbuild\pythoncore.vc 
xproj]
  inffast.c
c1 : fatal error C1083: Cannot open source file: 'C:\Users\gvanrossum\deepfreeze\externals\zlib-1
.2.11\inffast.c': No such file or directory [C:\Users\gvanrossum\deepfreeze\PCbuild\pythoncore.vc 
xproj]
  inflate.c
c1 : fatal error C1083: Cannot open source file: 'C:\Users\gvanrossum\deepfreeze\externals\zlib-1
.2.11\inflate.c': No such file or directory [C:\Users\gvanrossum\deepfreeze\PCbuild\pythoncore.vc 
xproj]
  inftrees.c
c1 : fatal error C1083: Cannot open source file: 'C:\Users\gvanrossum\deepfreeze\externals\zlib-1
.2.11\inftrees.c': No such file or directory [C:\Users\gvanrossum\deepfreeze\PCbuild\pythoncore.v 
cxproj]
  trees.c
c1 : fatal error C1083: Cannot open source file: 'C:\Users\gvanrossum\deepfreeze\externals\zlib-1
.2.11\trees.c': No such file or directory [C:\Users\gvanrossum\deepfreeze\PCbuild\pythoncore.vcxp 
roj]
  uncompr.c
c1 : fatal error C1083: Cannot open source file: 'C:\Users\gvanrossum\deepfreeze\externals\zlib-1
.2.11\uncompr.c': No such file or directory [C:\Users\gvanrossum\deepfreeze\PCbuild\pythoncore.vc 
xproj]
  zutil.c
c1 : fatal error C1083: Cannot open source file: 'C:\Users\gvanrossum\deepfreeze\externals\zlib-1
.2.11\zutil.c': No such file or directory [C:\Users\gvanrossum\deepfreeze\PCbuild\pythoncore.vcxp 
roj]
  dl_nt.c

Build FAILED.

(Followed by the same errors repeated.)

Second build:

Using py -3.9 (found 3.9 with py.exe)
Fetching external libraries...
bzip2-1.0.6 already exists, skipping.
sqlite-3.35.5.0 already exists, skipping.
xz-5.2.2 already exists, skipping.
Fetching zlib-1.2.11...
Fetching external binaries...
libffi-3.3.0 already exists, skipping.
openssl-bin-1.1.1l already exists, skipping.
tcltk-8.6.11.0 already exists, skipping.
Finished.

And then everything builds problem-free.